ANNAPOLIS, Md. — Six people remain unaccounted for after a fierce fire gutted a 16,000-square-foot mansion in Maryland's capital early Monday, a fire official said.

The fire was reported about 3:30 a.m. by an alarm monitoring company and a neighbor who saw flames at the mansion in Annapolis, said Capt. Russ Davies, spokesman for the Anne Arundel County Fire Department.

"From the family, we know who's unaccounted for," Davies said, though he declined to say who specifically could not be located. "If you look at the damage, you know, it would not be a stretch to think that if there were occupants that they did not survive the fire."

Earlier, Davies had said the home's occupants might have been out of town.

Teary-eyed neighbors gathered near the gate leading to the waterfront home. Two teddy bears and flowers were placed beside the gate leading to the home.

Online records of the Maryland Department of Assessments and Taxation list Don and Sandra Pyle as owners of the home, which the records say is their principal residence.

Don Pyle was named CEO of ScienceLogic in September. The company, based in Reston, Virginia, makes software for cloud computing.

Yama Habibzai, a spokesman for ScienceLogic, confirmed in an email to The Associated Press that there was a fire at Pyle's home early Monday and the case is under investigation. He said the company had no further comment.

Davies said the cause of the fire, which caused "multiple millions" of dollars in damage, has not been determined.

The fire was brought under control hours after it began by some 85 firefighters called in from several areas. Davies said because there was no fire hydrant in the area, firefighters shuttled water tankers to the site and stationed a fire boat at a pier near the property to bring in water.

Because of the intensity of the fire, the size of the house and the failure of the roof and floors, firefighters have been unable to search the structure, Davies said. He added that firefighters remained on the site beside the mansion, which had been reduced to its stone framework.

Agents with the Bureau of Alcohol, Tobacco, Firearms and Explosives also were on the premises Monday afternoon. Special Agent David Cheplak, a spokesman for the ATF's Baltimore field office, said local fire officials had asked for help.

"The Anne Arundel County Fire Department had asked for our assistance, and so we've activated our national response team, which is a team comprised of special agents, certified fire investigators from across the country, and they respond to large-scale fires nationally," Cheplak said.

Cheplak said there was no evidence at this point of foul play. He said the team was expected to arrive Tuesday.

According to a 2008 story in The Baltimore Sun, the Pyles' house, which was the site of a charity event, was described as looking like a castle, with mini-turrets, stonework and lion statues.

State property records said the two-story house was built in 2005 and had seven bedrooms and 7 ½ bathrooms. Its value was listed at $4.2 million.